Theoretical calculations of bite force in billfishes

Bite force is one of the most utilized performance measures due to its association with feeding and consequently fitness. Due to their large size and the concomitant increase in the adductor muscles it is expected that apex predators may rely on high biting performance allowing for increased dietary breadth. Billfish are apex marine predators characterized by the extreme elongation of the upper jaw bones forming a rostrum. This structure has been shown to facilitate prey capture and processing, but little is known as to whether this elongated structure has had an effect on biting performance in this group of enigmatic fishes. The goal of this study was to investigate bite force among five billfish species differing in size and rostrum morphology, and to determine the relationship between bite force and cranial anatomy. Because prey processing is partially decoupled from the oral jaws in these fishes, we hypothesized that bite force will be relatively low in billfishes compared to other fish, and that mass specific bite force will be inversely proportional to rostrum length. Anterior and posterior values of bite force ranged from 29 to 330 N and 99 to 1361 N, respectively among billfish species. Although absolute bite forces reached high values, mass specific bite force was among the lowest recorded for cartilaginous and bony fishes. Contrary to our hypothesis, rostrum length was not a predictor of bite force. The evolution of this novel structure, however, has been demonstrated to have a role in feeding, and the relatively low bite force in billfishes may indicate a lack of dependence on this performance metric for prey capture and processing.